Likelihood to Recommend
Riak's SOLR engine is fast, however if you have extremely high amount of queries in a very limited time range, it can fail in a bad way.
Riak is well suited as a key value store. It does exactly what it says it does. If you have well known buckets/keys, Riak is a great solution for a ton of different use cases. The lack of ability to search is somewhat problematic for other use cases requiring this ability.
Also, while the ability to store a variable array of data into a single bucket/key is extremely useful, if you have a use case requiring parts of that value to be updated independently, Riak does not support transactions so you open yourself up to contention issues if the data is being updated regularly in small portions. One solution to this is to use multiple bucket/key parts to store the data, which will remove the contention issue, but then you have to increase your Riak footprint which results in more buckets and can sometimes make things more difficult than needed. This has been a nagging issue we've had to deal with on multiple occasions.